PROJECT SUMMARY/ABSTRACT New and improved drugs are needed for human multiple sclerosis (MS) given that available treatments have only modest effects on the disease course. Current drugs demonstrate better efficacy than the previous first- line treatments (interferon- and glatiramer acetate), but have greatly increased toxicity and risk of serious life- threatening infections due to global immunosuppression. A preferred treatment approach would be a safe and well-tolerated biologic with a potent ability to inhibit key disease-associated mechanisms involved in the pathogenesis of MS. Our laboratory discovered and is developing partial MHC class II constructs (pMHC) as a possible immunotherapy for MS. pMHC containing the extracellular domains of the MS risk factor, HLA-DR2, linked covalently to the encephalitogenic myelin oligodendrocyte glycoprotein (MOG)-35-55 peptide (pDR2/MOG-35-55) can reverse CNS inflammation and clinical signs of MOG-peptide-induced experimental autoimmune encephalomyelitis (EAE) in DR2 transgenic mice. The same construct (termed RTL1000) was recently used successfully in a Phase 1 safety trial in MS patients. We have strong pre-clinical evidence to support the idea that RTL1000 can specifically target both peptide-specific and bystander T-cells reactive to other encephalitogenic myelin peptides. This inhibitory activity of the pMHC complexes requires binding of the 11 moiety to a specific receptor expressed predominantly on monocytes. We recently we discovered that pMHC can bind specifically to the invariant chain of MHC class II (CD74), down-regulating its expression on the monocyte cell surface and blocking the inflammatory effects of macrophage migration inhibitory factor (MIF) that is a key pathogenic cytokine for EAE and MS. We further demonstrated that the DR-1 domain is the major CD74 binding moiety of pDR2 constructs. Moreover, increased concentrations of DR-1 (in the absence of a bound antigenic peptide) could down-modulate CD74 levels on monocytes. These findings suggested that DR-1 alone might modulate MIF activity and treat EAE in an antigenic peptide-independent manner through its effects on CD74 expression. Indeed, our preliminary experiments demonstrated a significant treatment effect of DR-1on EAE. These data have important regulatory implications that enable the development of a new class of therapeutic DR-1-derived constructs for autoimmune and inflammatory conditions involving MIF signaling through CD74. We envision that modulation of CD74 by DR-1 through its CD74 binding region will provide significant therapeutic effects in a wide spectrum of inflammatory conditions even when the specificity of pathogenic T-cells has not been established, and that efficacy of treatment may be enhanced by attachment of specific antigenic peptides to the DR-1 moiety in conditions where pathogenic T- cell specificities are known or strongly suspected (eg. to myelin peptides in MS). Moreover, because the DR- 1 domain is present in all humans and thus would not be recognized as foreign, treatment using DR-1 constructs would not require HLA screening of potential recipients and could be used for treatment of MS subjects who do not express the HLA-DR2 risk factor. Our preliminary data suggest a significant increase in CD74 levels on monocytes in mice with EAE and subjects with MS, thus providing a compelling rationale for development off pMHC constructs that can inhibit MIF activity by down-regulating CD74 expression. In this application we will test the hypothesis that a specific segment of DR-1 binds to monocytes and decreases cell-surface expression of CD74 and MIF signaling that in turn blocks cell migration into the CNS and Teff-cell activation, resulting in reversal of clinical and histological signs of EAE. To address this hypothesis, we propose to 1) Identify the CD74-binding region within DR-1 and develop an optimal CD74-binding DR-1 construct; 2) Evaluate inhibitory effects of DR-1 constructs on monocyte and T-cell activation; and 3) Determine efficacy of DR-1 constructs for treatment of EAE induced by peptide-specific vs. non-specific (bystander) T-cell specificities.